222. Count Complete Tree Nodes

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最新推荐文章于 2025-05-24 17:38:15 发布

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本文链接：https://blog.youkuaiyun.com/weixin_57706549/article/details/128059764

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本文介绍如何利用递归和辅助函数在O(n)时间复杂度内解决给定完整二叉树根节点的问题，计算树中节点总数。通过理解完全二叉树结构并应用算法技巧，轻松实现高效的计数方法。

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Given the root of a complete binary tree, return the number of the nodes in the tree.

According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

Design an algorithm that runs in less than O(n) time complexity.

Input: root = [1,2,3,4,5,6]
Output: 6

Eg:
Input: root = [1,2,3,4,5,6]
Output: 6

Solution:

class Solution {
    public int countNodes(TreeNode node) {
        if (node == null) return 0;
        int leftNode = findNode(node.left);
        int rightNode = findNode(node.right);
        int result = leftNode + rightNode + 1;
        return result;
        // return findNode(root);
    }
    public int findNode(TreeNode node){
        if (node == null) return 0;
        int leftNode = findNode(node.left);
        int rightNode = findNode(node.right);
        int result = leftNode + rightNode + 1;
        
        return result;
    }
}

Hints:

1: Understand what is a complete binary tree.

2: Know how to return the number of nodes

3: make sure the parameter of the helper function